The present invention relates to a method and apparatus for preventing the formation of alkane hydrates in subsea oil and gas production equipment. More particularly, the invention relates to such a method and apparatus which relies on the use of a phase change material to maintain the produced well fluid above a predetermined temperature below which alkane hydrates will form.
Subsea oil and gas wells which are located at depths greater that 5,000 feet or at extreme latitudes are exposed to water which is typically just a few degrees above freezing. Although the well fluid is relatively hot as it flows through the subsea production equipment, the surrounding water will cool the fluid rapidly when the flow is interrupted for any length of time, such as by a temporary well shut down. If the well fluid is allowed to cool to below the hydrate formation temperature of the fluid, which averages about 70xc2x0 F. for typical hydrocarbons, alkane hydrates may form in the fluid which may block the flow paths through the production equipment.
In the prior art, thermal insulation materials have been used on the subsea equipment to retard the cooling of the fluid until flow can be restored or a hydrate inhibitor can be injected into the fluid. However, several inches of insulation are usually required to provide the necessary thermal barrier, and this affects the design and manufacture of the subsea equipment and adds significantly to the cost of the equipment. In addition, the hydrate inhibitor and the equipment required to inject it into the fluid are relatively expensive.
These and other disadvantages in the prior art are overcome by the present inventive method of preventing the formation of alkane hydrates in a subsea oil and gas production equipment component having at least one flow path through which a well fluid is permitted to flow. The method comprises the steps of providing a phase change material which has a melting point below the normal flow temperature of the well fluid but above the hydrate formation temperature of the well fluid, and positioning the phase change material in heat exchange relation with respect to the flow path. Thus, when the well fluid cools and the temperature of the phase change material drops to its melting point, the phase change material will solidify and its latent heat will be transferred to the well fluid to maintain the temperature of the well fluid above the hydrate formation temperature. The present invention can therefore be seen to provide a passive method for maintaining the temperature of the well fluid above its hydrate formation temperature without the need for bulky insulation or costly hydration inhibitors.
In accordance with another aspect of the present invention, a temperature control device is provided for preventing the formation of alkane hydrates in a subsea oil and gas production equipment component having at least one flow path through which a well fluid is permitted to flow. The temperature control device comprises a housing which is positioned in heat exchange relation with respect to the flow path and a phase change material which is disposed in the housing. The phase change material has a melting point which is below the normal flow temperature of the well fluid but above the hydrate formation temperature of the well fluid. Thus, when the well fluid cools and the temperature of the phase change material drops to its melting point, the phase change material will solidify and its latent heat will be transferred to the well fluid to maintain the temperature of the well fluid above the hydrate formation temperature. In one embodiment of the invention, the housing comprises a flexible thermoplastic material and the temperature control device further comprises means for securing the housing in heat exchange relation with respect to the flow path. In another embodiment of the invention, the flow path is defined by a first conduit, the housing comprises a second, concentric conduit having a larger diameter than the first conduit and the phase change material is disposed between the first and second conduits. In accordance with yet another embodiment of the invention, the temperature control device also comprises a thermal insulation material disposed around the housing. Accordingly, the temperature control device effectively maintains the temperature of the well fluid above its hydrate formation temperature without the need for bulky insulation or costly hydration inhibitors.
These and other objects and advantages of the present invention will be made more apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.